Understanding constant current drivers

Hi all

I've been building some prototype bike lights, using pic microcontrollers to control buckpuck drivers, driving luxeon k2s.

Problem is, I feel that I'm hitting the limits of this approach and I now want to investigate the possibilities of building my own driver circuits.

Can anyone suggest a good reference / starting point for this. I have good basic electronics knowledge, but I do not understand the theory / practice of these devices.

Thanks in advance

Pete

Sounds like you've already played with the basic design as a constant current driver has the same components as you've already used: a u-controller and a driver chip. The driver chip can be a DC-DC converter or an LED driver. It just has to have a PWM (Pulse Width Modulation) or PFM (Pulse Frequency Modulation) input to control it. The u-controller will provide the signal. You need a sense resistor in the path of the output current to feed the Vsense back to the uC. The uC will then just adjust the PWM or PFM signal to the driver/DC-DC chip to adjust the Vo. It tries to maintain the Vsense which in turn maintains Io. A tight tolerance sense resistor would provide more accurate regulation.

jsr

Thanks for that, it certainly starts to make things a whole lot clearer, and yes, it is similar to what I've been doing, as I've been using pwm into the buck pucks to control brightness. Do you know of anywhere I might find some schematicsor more detailed info to save doing the whole thing from scratch.

Pete

Sorry, don't know of any specific ones. I'd suggest looking up some LED drivers and checking the sample applications or circuits on their datasheets. The specific design depends on the driver chip you'll be using.

Some LED driver chips are already constant current and have an internal op-amp to for sense and adjustments. However, using those limits you to the output current of those driver chips. But, making one on your own would require more work. Depends on what you need. Since you're driving K2s, it sounds like you want to run a lot of current and most constant current LED drivers are fairly low current.

Since your setup already runs a uC and buck with PWM/PFM input, you can likely just send a sense signal back to the uC.

You could warm yourself up with this:


http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf